Floating or submersible facilities, such as ships or wind energy systems for example, can be exposed to highly variable operating conditions. Such highly variable operating conditions may be for example different thrust and speed requirements, as are present when a ship travels through ice and freely. When travelling through ice a relatively high level of thrust is required at low speed (or rate of advance) and when travelling freely a relatively low level of thrust is required at high speed (or rate of advance). Highly variable operating conditions can for example also be due to very different load states or usage profiles (e.g. patrol speed and cruise speed in the case of a ship). Another example is the (load-free) switching of a propeller from sail position to propulsion position in the case of a sailing ship.
When ships are operating in ice the entire output of the drive machine is generally required for forward movement. The output of the machine here is determined by the product of speed and torque. When travelling through ice, in order to be able to supply the maximum output to overcome the resistance produced by the ice, the speed of the motor should be selected so that a maximum output is achieved. In the case of an electric motor this is generally achieved at maximum speed (without field-weakening mode). This means that the propeller is designed or set for a low rate of advance in the water at maximum motor speed (without field-weakening mode). For free travel, in other words travel on open sea or lake, it is then barely possible to increase speed, particularly in the case of permanent-field electric motors. This means that only low speeds can be achieved even during free travel.
To resolve this problem DE 101 357 11 A1 discloses a drive train for a shaft system of an ice-breaking ship or one that travels in ice, wherein an internal combustion engine is present, to the output shaft of which a step-down gearing system is connected, which acts on a propeller shaft on the output side, and a rotor of an electric generator connected to the output shaft of the internal combustion engine is also present. When traveling through ice the propeller pitch is scaled back and the electric load of the generator is reduced. These measures prevent the motor speed dropping below a specified value.
A nacelle drive is known from DE 198 347 36 A1, in which a propeller casing is mounted around the drive propeller of the nacelle drive for an ice-free navigation period. The propeller casing is removed during navigation under ice conditions. The power consumption of the propeller is kept constant in casing mode and casing-free mode. To this end the blade pitch of the propeller is enlarged during free travel (i.e. casing mode) and reduced during ice travel (i.e. casing-free mode).
The pitch adjusting facility for adjusting the pitch of the propeller blades in a nacelle drive generally comprises—as disclosed for example in WO 2005/021374 A1—a hydraulic drive and regulation facility. However this requires the propeller shaft to be configured as a hollow shaft and hydraulic oil to be introduced into the control and drive lines to the hydraulic adjusting hub disposed in the hollow chamber of the hollow shaft. However there is no space for this in nacelle drives, in particular when an electric drive motor is accommodated in the nacelle housing to drive the propeller shaft and when two propellers are used. There is also the risk of an oil leak in the nacelle housing.